Sebastian withdrew from the Senate at the start of the Civil War and was later formally expelled by the Senate. He took no active part in the Confederate government, and was reinstated by a posthumous resolution in 1877.

Sebastian was born in Centreville, Tennessee in 1812; sometime around 1834 he graduated from Columbia College, also in Tennessee, and began studying law. In 1835 he was admitted to the bar and began practice in Helena, Arkansas; later he became a cotton planter. From 1835 to 1837 he was a prosecuting attorney; he served as a circuit court judge from 1840 to 1843, in which year he was made an associate justice of the Arkansas Supreme Court. In 1846 he became a member of the state Senate, serving as its president until 1847. Also in 1846 he served as a presidential elector on the Democratic ticket. In 1848, upon the death of Chester Ashley, he was appointed to the United States Senate, subsequently being elected in his own right. He was reelected in 1853 and 1859; in July of 1861 he was expelled for his support of the Confederacy. During his time in the senate, he served as the chair of the Committee on Manufactures, as well as on the Committee on Indian Affairs Upon his ejection from the Senate, Sebastian returned to Helena, where he lived for the duration of the Civil War and practiced law. After federal troops occupied Helena, Ark., he moved to Memphis, Tennessee, in 1864 and resumed the practice of law; he died there on May 20, 1865, and is buried in a private family cemetery in Phillips County. Sebastian County, Arkansas is named for him. In 1877, the Senate revoked the resolution of expulsion which they had passed upon Sebastian, and consequently paid the compensation due to Sebastian’s children.